escaping tricky string to CSV format

I use this code and it has always worked:

/// <summary>
/// Turn a string into a CSV cell output
/// </summary>
/// <param name="str">String to output</param>
/// <returns>The CSV cell formatted string</returns>
public static string StringToCSVCell(string str)
{
    bool mustQuote = (str.Contains(",") || str.Contains("\"") || str.Contains("\r") || str.Contains("\n"));
    if (mustQuote)
    {
        StringBuilder sb = new StringBuilder();
        sb.Append("\"");
        foreach (char nextChar in str)
        {
            sb.Append(nextChar);
            if (nextChar == '"')
                sb.Append("\"");
        }
        sb.Append("\"");
        return sb.ToString();
    }

    return str;
}